The disclosure relates generally to communications systems and, more particularly, to multiple frequency capable phase locked loops (PLLs) which may be used in communications systems, including wireless communications systems (WCSs), such as wireless distribution systems (WDSs), distributed antenna systems (DASs), remote radio head (RRH) systems, and small radio cell systems.
Communications circuits have evolved from cumbersome wire-based systems that could only transmit simple codes to sophisticated wireless systems that support high-bandwidth multimedia communication. Early systems relied on a super-heterodyne receiver. Such super-heterodyne receivers included several tuned circuits, which increased the complexity of the device. As early as 1932, British researchers looking for an alternative to the structure of the super-heterodyne receiver created a direct-conversion or homodyne receiver. The homodyne receiver used a local oscillator to tune to a desired input frequency and provide an output which was multiplied with the input signal to provide the original modulation information. It was rapidly discovered that the local oscillator would drift in frequency, and thus, an automatic correction signal was applied to the oscillator, maintaining it in the same phase and frequency of the desired signal. This automatic correction signal was generated by a circuit known as a PLL. Applications for PLLs have proliferated and can be found in communications circuits for a variety of functions such as frequency mixing, modulation, demodulation, synchronization, frequency synthesis, signal recovery, and the like.
The simplest PLL circuits are a phase comparator, a loop filter, and a voltage controlled oscillator (VCO) that provides a feedback signal to the phase comparator. The phase comparator compares the feedback signal to the input signal and generates a control signal for the VCO. As with many feedback systems, it takes time for the control signal to adjust the VCO to the point where the output of the VCO is in phase with the input signal. This time may sometimes be referred to as the time required to settle and lock.
While there are many communications systems in which a PLL may be used, the demand for wireless communications systems that include such PLLs is increasing. That is, wireless customers are increasingly demanding digital data services, such as streaming video signals. At the same time, some wireless customers use their wireless communications devices in areas that are poorly serviced by conventional cellular networks, such as inside certain buildings or areas where there is little cellular coverage. One response to the intersection of these two concerns has been the use of a WDS or a DAS. WDSs include remote units configured to receive and transmit communications signals to client devices within the antenna range of the remote units. WDSs can be particularly useful when deployed inside buildings or other indoor environments where the wireless communications devices may not otherwise be able to effectively receive radio frequency (RF) signals from a source.
In this regard, FIG. 1 illustrates distribution of communications services to remote coverage areas 100(1)-100(N) of a WDS 102, wherein ‘N’ is the number of remote coverage areas. These communications services can include cellular services, wireless services, such as Common Public Radio Interface (CPRI), RF identification (RFID) tracking, Wireless Fidelity (Wi-Fi), local area network (LAN), and wireless LAN (WLAN), wireless solutions (Bluetooth, Wi-Fi Global Positioning System (GPS) signal-based, and others) for location-based services, and combinations thereof, as examples. The variety of services may dictate that each service have its own transceiver with accompanying PLL. The remote coverage areas 100(1)-100(N) may be remotely located. In this regard, the remote coverage areas 100(1)-100(N) are created by and centered on remote units 104(1)-104(N) connected to a head-end equipment (HEE) 106 (e.g., a head-end controller, a head-end unit (HEU), or a central unit). Each such remote unit 104(1)-104(N) may include one or more PLLs. The HEE 106 may be communicatively coupled to a signal source 108, for example, a base transceiver station (BTS) or a baseband unit (BBU). In this regard, the HEE 106 receives downlink communications signals 110D from the signal source 108 to be distributed to the remote units 104(1)-104(N). The remote units 104(1)-104(N) are configured to receive the downlink communications signals 110D from the HEE 106 over a communications medium 112 to be distributed to the respective remote coverage areas 100(1)-100(N) of the remote units 104(1)-104(N). In a non-limiting example, the communications medium 112 may be a wired communications medium, a wireless communications medium, or an optical fiber-based communications medium. Each of the remote units 104(1)-104(N) may include an RF transmitter/receiver (not shown) and a respective antenna 114(1)-114(N) operably connected to the RF transmitter/receiver to wirelessly distribute the communications services to client devices 116 within the respective remote coverage areas 100(1)-100(N). The remote units 104(1)-104(N) are also configured to receive uplink communications signals 110U from the client devices 116 in the respective remote coverage areas 100(1)-100(N) to be distributed to the signal source 108. The size of each of the remote coverage areas 100(1)-100(N) is determined by an amount of RF power transmitted by the respective remote units 104(1)-104(N), receiver sensitivity, antenna gain, and RF environment, as well as by RF transmitter/receiver sensitivity of the client devices 116. The client devices 116 usually have a fixed maximum RF receiver sensitivity, so that the above-mentioned properties of the remote units 104(1)-104(N) mainly determine the size of the respective remote coverage areas 100(1)-100(N).
With reference to FIG. 1, the HEE 106 includes electronic processing devices, for example a field-programmable gate array (FPGA), a digital signal processor (DSP), and/or a central processing unit (CPU), for processing the downlink communications signals 110D and the uplink communications signals 110U. Likewise, each of the remote units 104(1)-104(N) also employs electronic processing devices for processing the downlink communications signals 110D and the uplink communications signals 110U. Further, the communications medium 112 is only able to carry the downlink communications signals 110D and the uplink communications signals 110U up to a maximum bandwidth. Collectively, the processing capabilities of the electronic processing devices in the HEE 106, the processing capabilities of the electronic processing devices in the remote units 104(1)-104(N), and the maximum bandwidth of the communications medium 112 provide the system resources available in the WDS 102. It may be desirable to utilize the system resources in the WDS 102 to improve overall performance of the WDS 102.
Within a WDS such as the WDS 102, PLLs may be used to reconstruct a clock signal at the remote units 104(1)-104(N). In some such systems, the same clock reconstruction circuit may support two different clock frequencies for two different applications (e.g., a 122.88 megahertz (MHz) clock for CPRI applications and a 156.25 MHz clock for Ethernet applications). Since the VCOs used for these applications are typically narrowband, a single VCO cannot be used to support both frequencies. Conventional solutions use two PLLs, one for each clock frequency. While conventional solutions are adequate for situations where the remote unit is powered and/or there is ample space, there may be instances where power savings or space constraints make such duplicative solution a burden.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinency of any cited documents.